The present invention relates to an active matrix liquid crystal display and, more particularly, to an active matrix liquid crystal panel which reduces the number of steps of the manufacturing process for the panel and realizes a high yield at a low cost.
Display devices of, e.g., electroluminescence, light-emitting diode, plasma, and liquid crystal types have display units, which can be made low-profile, and are promising in application to display for a television set, a measurement equipment, an office equipment, a computer, and the like. Of these devices, a liquid crystal display with a TFT liquid crystal panel, having a matrix array using thin film transistors (TFTS) as switching elements, can realize full color display and low power consumption.
Materials for such a switching transistor are crystalline Si, polycrystalline Si, amorphous Si, CdSe, Te, CdS, and the like. Of these materials, particularly the polycrystalline and amorphous semiconductors can be processed by a thin film technique in a low-temperature process. For this reason, these semiconductors can be used as a material which forms the active matrix element of a switching transistor even on a substrate made of a material, such as glass, which must be processed at a relatively low temperature. By employing this technique, large-area liquid crystal displays can be mass-produced at a low cost.
FIGS. 31A to 31F show an example of a method of manufacturing a conventional active matrix liquid crystal display panel using an amorphous silicon (a-Si) film as an active layer.
First, an undercoat layer 102 consisting of, e.g., SiOx is formed by sputtering on a transparent insulating substrate 101 such as a glass substrate. A conductive layer consisting of a refractory metal such as Cr or an Mo--Ta alloy is formed on the undercoat layer 102. The refractory metal conductive layer is patterned to form a gate electrode 103 and a pad portion 104 serving as a lead-out portion (FIG. 31A; first mask step).
The gate electrode 103 and the pad portion 104 are covered with an insulating film 105 of, e.g., SiNx. After an a-Si film 106 is stacked as an active layer on the insulating film 105 at a position above the gate electrode, an n.sup.+ -type a-Si film 107 is further stacked as an ohmic-contact layer. These films are etched into a predetermined pattern (FIG. 31b; second mask step).
A transparent electrode film 108 consisting of, e.g., ITO and serving as a pixel electrode is formed into a predetermined pattern on the insulating film 105 (FIG. 31C; third mask step). Further, the insulating film 105 is removed at the lead-out portion of the gate electrode 103, e.g., the pad portion 104 by etching (FIG. 31D; fourth mask step).
A source electrode 109a and a drain electrode 109b are formed on the n.sup.+ -type a-Si film 107 with a predetermined distance therebetween. Using the source and drain electrodes 109a and 109b as part of a mask, the n.sup.+ -type a-Si film 107 between the source and drain electrodes 109a and 109b is removed by etching to form a TFT (FIG. 31E; fifth mask step). To improve the durability, a protection film 110 of, e.g., SiNx is deposited on the TFT. The protection film 110 is removed at the lead-out portion of the electrode such as the pad portion 104 (FIG. 31F; sixth mask step), completing the active matrix panel.
In the method of manufacturing an active matrix panel, the number of mask steps is as large as six, as described above, resulting in an increase in manufacturing cost. A low-cost active matrix panel cannot be obtained.
In removing the n.sup.+ -type a-Si film 107 by etching, the a-Si (amorphous silicon) film 106 is also etched, so that the a-Si film must be made thick. In general, an a-Si film having a thickness of about 200 to 300 nm is used. The film formation process takes a long time, the productivity is degraded, and management of the etching step with respect to the n.sup.+ -type a-Si film is complicated.
On the other hand, a method disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 6-18215 is also available. According to this method, a gate electrode is selectively formed on an insulating substrate, part of the lead-out portion of the gate electrode is masked, and a gate insulating film, an a-Si film, an n.sup.+ -type a-Si film, and a metal film are continuously deposited. Then, the a-Si film, the n.sup.+ -type a-Si film, and the metal film are patterned into almost the same shape. A transparent conductive film is deposited on the entire surface. This transparent electrode is patterned into the wiring or interconnect shapes of source and drain electrodes also serving as pixel electrodes. The metal film and the n.sup.+ -type a-Si film are selectively removed using the transparent conductive film pattern as part of a mask, completing the active matrix liquid crystal display.
In the method of manufacturing an active matrix liquid crystal display, the gate insulating film, the a-Si film, the n.sup.+ -type a-Si film, and the metal film must be continuously deposited upon masking the gate contact electrode with the metal mask or the like. As a result, a film on the metal mask may peel off to greatly decreased the yield. Particularly when a large number of active matrix liquid crystal displays are cut from one substrate, a metal mask must be set at the central portion of the substrate, greatly decreasing the yield.
A method of lift-off using a resist or the like instead of a metal mask is another option. However, since the substrate temperature must be elevated in depositing a gate insulating film, an a-Si film, and an n.sup.+ -type a-Si film, a normal resist cannot be used. Even if the substrate temperature during deposition is lowered (up to 130.degree. C., the film lifted off and the like may reattach to the substrate in the lift-off step, resulting in a decrease in yield.